Cancer is considered to be a serious and pervasive disease. The National Cancer Institute has 20 estimated that in the United States alone, 1 in 3 people will be struck with cancer during their lifetime. Moreover approximately 50% to 60% of people contracting cancer will eventually succumb to the disease. Hence, since the establishment the National Cancer Institute in the early 1970's, the amount of resources committed to cancer research has dramatically improved.
Although cancer is commonly considered to be a single disease, it actually comprises a family of diseases wherein normal cell differentiation is modified so that it becomes abnormal and uncontrolled. As a result, these malignant cells rapidly proliferate. Eventually, the cells spread or metastasize from their origin and colonize other organs, eventually killing their host. Due to the wide variety of cancers presently observed, numerous strategies have been developed to destroy cancer within the body. One such method utilizes cytotoxic chemotherapeutics. These compounds are administered to cancer sufferers with the objective of destroying malignant cells while leaving normal, healthy cells undisturbed. Particular examples of such compounds include 5-fluorouracil, cisplatin, and methotrexate.
Combretastatin A-4 was initially isolated from stem wood of the African tree combretum caffrum (Combretaceae), and found to be a potent inhibitor of microtubulin assembly. Moreover, combretastatin A-4 was found to be significantly active against the US National Cancer Institute's (NCI) murine L1210 and P338 lymphocytic leukemia cell lines. In addition, combretastatin A-4 was found to compete with combretastatin A-1, another compound isolated from Combretum caffrum, as an inhibitor of colchicine binding to tubulin. It has also been determined to retard strongly the VoLo, DLD-1 and HCT-15 human-colon cancer (ED50<0.01 (μg/ml) cell lines, and to be one of the stronger anti-mitotic agents found among the Combretum caffrum constituents (U.S. Pat. No. 4,996,237).
Consequently, research has been conducted to determine the efficacy of combretastatin A-4 as a chemotherapeutic in treating a variety of human cancers. Unfortunately, combretastatin A-4 is essentially insoluble in water. This characteristic has significantly interfered with the development of pharmaceutical compositions comprising combretastatin A-4. In order to increase its solubility as well as its efficacy, efforts have been made to create prodrug derivatives of combretastatin A-4 that will regenerate combretastatin A-4 in physiological conditions. For example, Koji Ohsumi et al. describe the synthesis of amino acid HCl prodrugs of a combretastatin analog, wherein an amino acid salt is attached to the amino group of a combretastatin derivative containing a basic amino group [the derivatives are described in Ohsumi et al, Anti-Cancer Drug Design, 14, 539-548 (1999)]. Although such prodrugs may have an increased solubility compared to native combretastatin A-4, they possess an inherent limitation in that the regeneration of combretastatin A-4 is dependent upon endogenous aminopeptidase in the blood of a subject to whom the prodrug is administered.
The free acid of combretastatin A-4 phosphate (“CA4P free acid”) which has the following structure:
exists as an oily mass. The CA4P free acid is intrinsically Very Slightly Soluble (by USP definition) at 25° C. in water, with the aqueous solubility increasing with an increase in pH. It has two acidic groups with pKa values of 1.2 and 6.2, which are amenable to salt formation. As there are practical issues with handling CA4P free acid due to its physical state, identification of a crystalline, stable salt form of this compound is desirable.
Attempts at derivatizing combretastatin A-4 have involved forming salt derivatives of combretastatin A-4 phosphate (salt derivatives of “CA4P”). Particular examples of such salts are set forth in U.S. Pat. No. 5,561,122. Although these prodrug salts possess greater solubility than native combretastatin A-4, they can also possess inherent drawbacks, such as hygroscopicity.
Hygroscopicity is one of several important criteria in the selection of a salt. See K. Morris et al., “An Integrated Approach to the Selection of Optimal Salt Form for a New Drug Candidate”, Int. J. Pharm., 105, 209-217 (1994). The extent of hygroscopicity for any drug substance has significant impact on its handling and stability over the lifetime of the drug product.
Accordingly, what is needed are novel and useful combretastatin A-4 prodrug salts with favorable physiochemical properties, and which increase the solubility, and preferably efficacy, of combretastatin A-4 in treating a wide variety of neoplastic disorders.
What is also needed are novel and useful combretastatin A-4 prodrug salts which readily regenerates native combretastatin A-4 in vivo and do not produce unwanted or potentially deleterious side products when undergoing regeneration.
The citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.